staging: dgap: remove useless a variable within board_t

The use_interrupts is used only in dagp_request_irq() for checking
a value from user config file. It doesn't need in board_t struct.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>